The prior art discloses externally excited electrical generator in an on-board electrical system of a motor vehicle and methods including setting the excitation voltage to regulate an actual output voltage of the generator at a predetermined setpoint output voltage of the generator. The apparatuses known for this purpose usually comprise:                a drivable DC/DC converter supplied from the on-board electrical system, for generating a (variable) excitation voltage for the generator, and        a comparison device for comparing an actual output voltage of the generator with a predetermined setpoint output voltage of the generator and for outputting a deviation signal (as drive signal) to the DC/DC converter to thereby regulate the actual output voltage at the setpoint output voltage.        
The generator is usually rotationally driven by mechanical coupling of the generator to an internal combustion engine (for example petrol or diesel engine) of the motor vehicle. The internal combustion engine can provide a rotary power used directly for driving the motor vehicle in question, or else serving, for example, as a so-called “range extender” of a series hybrid motor vehicle solely to drive the electrical generator.
During the abovementioned regulation of the generator output voltage, an actual output voltage is continuously detected as a control variable, compared with the predetermined setpoint output voltage as a reference variable, and influenced as part of the process of matching the actual output voltage to the setpoint output voltage by setting the excitation voltage as stated.